In general, scan shift failures are difficult to debug. Usually we use the compressed-mode chain test or scan capture-based chain diagnosis to ascertain the small chain segment or the position of the sequential element in the chain that is the cause of the failure. This method of diagnosis works well when the failures are static and limited to a chain segment, but fails to give results when the failure is caused by intra chain or inter chain segment interactions. This paper presents a scenario in which the root cause of the chain failures was due to interaction between chains. We call it the “invisible flaky chain defect” because -- although we were able to replicate failure at a test point (voltage-frequency) on the Shmoo -- the failing cycles changed from run to run.

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